The present invention relates generally to the field of splices for electrical conductors and, more specifically, to a splice configuration particularly useful with superconducting cable-in-conduit conductors of the sort used in superconducting magnetic energy storage systems.
Superconducting magnetic energy storage (SMES) systems are capable of storing large amounts of electrical energy in a DC magnetic field for indefinite periods. Power from a utility grid or other power source such as a wind turbine or solar plant can be stored until needed, then returned to the utility grid or any specific application at any time. Utility applications include load leveling, spinning reserve, transmission system stability and reliability and voltage/power factor correction.
A SMES system often utilizes a cable-in-conduit conductor, which includes a superconducting alloy, wound into a large diameter solenoidal coil. The conductor is cooled to a temperature which will allow for superconductivity of the coil. With present commercial superconducting materials, the coil must be cooled with supercritical or liquid helium to a temperature approaching xe2x88x92456xc2x0 F. (1.8xc2x0 K.).
Typical SMES coils frequently are very large in diameter, often having diameters of over 10 feet. Because of the length of the coils, it is necessary to splice lengths of cable to form the continuous coil. These splices must have electrical and physical properties that do not degrade the performance of the coil.
With the regular conductive materials (i.e., copper or aluminum) used at ambient temperatures, two cables are mechanically fastened or soldered together. Typical of the prior art techniques for joining two conductors together are the crimp rings disclosed by Bennett in U.S. Pat. No. 3,231,964, and soldering as described by McIntosh et al. in U.S. Pat. No. 3,517,150.
Superconductors are materials, often metals or ceramics, that lose all resistance when cooled below a critical temperature. Many materials have superconducting capabilities, although most materials will only superconduct at temperatures approaching 0xc2x0 K. At present, the most practical of these materials are superconductive magnets that are cooled to a temperature at or near the boiling point of helium (4.20 K.). Given these extreme conditions, cables connected to such superconducting magnets must be connected via special splices that are not normally encountered under xe2x80x9cregularxe2x80x9d conditions (as mentioned above).
Instead, NbTi alloys and the compound Nb3Sn are used in this regard. The most common method of splicing these alloys has been the lap splice, where the cable ends are overlapped and soldered together. Such soldered lap splices exhibit relatively high resistance which leads to excessive local heating and ultimately raises the spliced superconductors above the critical superconducting temperature, thereby causing a loss of superconductivity. The loss of superconducting properties can also occur due to the frictional heating caused by motion of a poorly supported current-carrying superconductor in the presence of a magnetic field, which results in a force on-the superconductor. In either case, the reduction or loss of superconductivity is one of the biggest problems encountered in SMES systems.
A number of highly specialized methods have been developed in order to connect ends of superconductor cables without interposing a high resistance material, such as solder, there-between. For example, U.S. Pat. No 4,794,688 proposes to overlap the strands of each cable and then crimp the strands together. However, this approach is not effective with many superconductor cables, and it only provides a mechanical joint that may have insufficient strength for some applications.
Alternatively, multi-filament cable ends have been joined by intertwining the superconductor filaments, heating to a diffusion temperature then crimping a sleeve over the connection as described by Smathers in U.S. Pat. No. 5,111,574 and Jones in U.S. Pat. No. 4,631,808. However, this complex process would be too difficult to consistently accomplish for a multi-stranded cable-in-conduit superconducting cable because of the number of times the process would need to be repeated to form a useful SMES device.
A cable-in-conduit conductor splice has also been developed. Specifically, a spliced sub-cable region is filled with solder, as described by Dew et al. in U.S. Pat. No. 5,600,095. Such solder filled sub-cable splices are prone to excessive local heating because the superconductor sub-cable surface is deprived of direct contact with coolant, such as liquid helium. Thus, these splices are inadequate for SMES applications.
Given the above, a continuing need exists for a simple but effective method of splicing many cable-in-conduit superconducting cables together to form a longer cable suitable for large magnet coils or other applications without increasing electrical resistance, decreasing coolant surface area, or otherwise degrading the electrical and physical properties of the coil.
It is an object of the present invention to provide a low resistance method and apparatus for splicing two separate cable-in-conduit superconductors to each other. Each cable-in-conduit superconductor has a plurality of sub-cable units, and each sub-cable unit has a plurality of sub-cable wire assemblies (each wire assembly can be multiple, stranded superconducting elements, such as niobium-titanium wires encapsulated by copper). The aforementioned splicing method and apparatus mates the sub-cable units, and the sub-cable wires thereof, to one another in manner that also allows for ease of construction and in-situ cooling of the surfaces involved. The overall combination will provide enhanced performance of the spliced connection relative to the previously known methods/apparatus described above. Moreover, it is expected that the present invention will have particular utility in the field of SMES systems.
Accordingly, a splicing apparatus comprises first and second superconducting cable-in-conduit units. Both superconducting cable-in-conduit units have a plurality of sub-cable units wherein the sub-cable unit comprises a plurality of sub-cable wires. Means for arranging the individual sub-cable units of the first superconducting cable-in-conduit unit to be in close proximity to the individual sub-cable units of the second superconducting cable-in-conduit unit is provided. The individual sub-cable wires of each unit are enclosed by means for mating every sub-cable wire from the first unit to that of the second unit in a fashion that allows for an electrical connection. The means for mating is fluidically connected to a means for cooling the sub-cable wires contained therein. Optionally, means for properly positioning each sub-cable unit proximate to the means for dividing may be included. The entire assembly may be contained in a housing which includes a coolant port.
A method for splicing superconductors is also described. First, a pair of superconducting cables is provided, wherein each superconducting cable comprises a plurality of sub-cable units and wherein each sub-cable unit comprises a plurality of sub-cable wires. A sealed housing unit is also provided. The sub-cable units are arranged so that the sub-cable units of the first superconducting cable are placed in close proximity to the sub-cable units of the second superconducting cable. Next, the sub-cable wires of each respective sub-cable unit are electrically joined and placed within the housing. Lastly, the joined wires are then cooled.
The aforementioned apparatus and method find particular applicability in the field of superconductive electrical connection devices. In particular, this invention is expected to improve performance of the superconductor in which the invention is incorporated or practiced.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.